If I have a string and wish to capture a possibly empty string (and subsequently check if indeed it was empty) how can I do this? My regular expression works when the captured section is not empty and on first impressions it might seem to work in case the string is empty since Console.WriteLine(second); produces no output.
string notEmpty = "abc|CaptureMeButICanBeEmpty|def";
string isEmpty = "abc||def";
string regEx = @"^\w+\|([^|]+|)\|\w+$";
var match = Regex.Match(notEmpty, regEx);
var match2 = Regex.Match(isEmpty, regEx);
string first = match.Groups[1].Value;
Console.WriteLine(first); // CaptureMeButICanBeEmpty
string second = match2.Groups[1].Value;
Console.WriteLine(second); // No Output
Console.WriteLine(match.Groups.Count); // 2
Console.WriteLine(match2.Groups.Count); // 2
However, it is impossible to know if second is empty because the capture group was empty or simply because there was no second capture group at all.
That is to say:
string third = match2.Groups[2].Value;
Console.WriteLine(second); // No Output
also produces no output, since the capture group is non-existent, and testing with something like
Console.WriteLine(string.IsNullOrEmpty(second));
Console.WriteLine(string.IsNullOrEmpty(third))
does not work either as both return true (I expected perhaps string.IsNullOrEmpty(second) to return true but string.IsNullOrEmpty(third) to throw an index out of bounds exception or something similar.
I realise I can use the result of match.Groups.Count to determine how many capture groups there should be in the sample code [note: C# defines Groups[0] as the entire match] but I do not want to have to rely on this.
Your attempt to check group count is wrong because the number of groups is set by (or in) the regex pattern: the number of groups is always the amount of capturing groups + 1 (for the whole match). So, if you check match.Groups.Count it will always give you 2 because ^\w+\|([^|]+|)\|\w+$ contains a single capturing group.
Now, it is not possible to use an optional pattern inside an obligaotry capturing group to check if a group matched or not, because the group value will be initialized with an empty string regardless, and the group .Success property will then be set to true.
The only way out here is to make the capturing group pattern optional and then check the match group Success property, and if it is true, get the string value of the group, else, assign the default value of your choice.
Here is a C# demo:
string notEmpty = "abc|CaptureMeButICanBeEmpty|def";
string isEmpty = "abc||def";
string regEx = @"^\w+\|([^|]+)?\|\w+$";
var match = Regex.Match(notEmpty, regEx);
var match2 = Regex.Match(isEmpty, regEx);
var first = match.Groups[1].Success ? match.Groups[1].Value : "No ouptut";
Console.WriteLine(first); // => CaptureMeButICanBeEmpty
var second = match2.Groups[1].Success ? match2.Groups[1].Value : "No ouptut";
Console.WriteLine(second.ToString()); // => No Output
Well, you may actually perform any actions you need if you use an if block after checking .Success property.